There is known an image scanner that emits light of red, green and blue in a lighting order of respective colors, that moves, in a sub-scanning direction, a reader having a line sensor extending in a main scanning direction, and that reads an image on an original document for each scanning line of red, green and blue.
When reading a monochromatic or intermediate color image with gradations, a known image scanner reads the image on the original document by emitting light alternately between a first lighting order in which light of red, green, and blue is emitted in this order and a second lighting order that is opposite to the first lighting order (that is, the order of blue, green, and red), in order to prevent the occurrence of coloring to a particular color.